Nitrogen limitation in Alpine lakes
Presenta: Fabio Lepori, (Institute of Earth Sciences, University of Applied Sciences and Arts of Southern Switzerland, Canobbio, Switzerland)
In the Alps, like in other mountain regions across the world, N enrichment because of atmospheric deposition is a major environmental concern. What are the effects on microphytobenthos and grazers in high‐elevation lakes? To find out, we conducted nutrient and grazing assays in two near‐natural lakes in the French Alps. In each lake, nutrient‐diffusing substrates, placed within cages that allowed or prevented access by herbivore invertebrates, were deployed for approximately three weeks during the summer of 2012. We tested the hypotheses that: [H1] the lakes are N limited, [H2] N enrichment alters microphytobenthos taxonomic composition, and [H3] N enrichment alters grazing pressure by invertebrates. Supporting H1, in both lakes, by the end of the assays, substrata enriched with N had greater biomass of microphytobenthos than unenriched controls. Supporting H2, the taxonomic composition of the microphytobenthos differed between N‐enriched substrata and controls. The taxa responsible for the differences varied, with N enrichment favoring the cyanobacteria Anabaena and the green algae Mougeotia in one lake and the diatoms Gyrosigma and Diatoma in the other. In contrast, H3 was not supported, as N enrichment and grazing had no interactive effect on microphytobenthos biomass. Our assays indicate that N enrichment in Alpine lakes can alter the accrual and composition of microphytobenthos, which likely contribute most primary production in these environments. Although grazing was not affected during our assays, long‐term effects on consumers remain possible, and these will be investigated next.
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